Olefin epoxidation is an industrially important reaction, especially the epoxidation of propylene to propylene oxide. In 1992, 7.2 billion pounds of propylene oxide were is produced, with 50% being synthesized via propylene epoxidation with t-butylhydroperoxide or ethylbenzene hydroperoxide. The catalysts currently used in these processes are either homogeneous molybdenum compounds or heterogeneous silica-supported titania materials.
Extensive spectroscopy studies of silica-supported titanium oxides have suggested that the active centers in these catalysts are the isolated surface titanium atoms that form the O.dbd.Ti(OSi).sub.2 structure with the silica framework rather than bulk TiO.sub.2 particles. Bulk TiO.sub.2 particles appear to be undesirable in that they catalyze the non-selective decomposition of hydroperoxides (X. Gao, I. E. Wachs, "Titania-Silica as Catalysts: Molecular Structural Characteristics and Physico-Chemical Properties," Catal. Today, 1999, 51, 233-254).
To prevent the formation of bulk TiO.sub.2 phases in heterogeneous titanium-silica catalysts, the isomorphous substitution of Si (IV) by Ti (IV) in zeolites has been investigated. These studies resulted in the successful synthesis of a number of titanium silicates, including TS-1, TS-2, Ti-beta, Ti-ZSM-48 and Ti-MCM-41. Spectroscopic studies have confirmed the presence of framework titanium atoms in these silicates and the absence of bulk TiO.sub.2 particles. Further investigations have also demonstrated catalytic activities of these titanium silicates in olefin epoxidation. For example, U.S. Pat. No. 4,833,260 discloses the use of TS-1 in the epoxidation of propylene with hydrogen peroxide. In addition, U.S. Pat. Nos. 5,374,747 and 5,621,122 disclose epoxidation of olefins with hydrogen peroxide or organic hydroperoxides over a crystalline molecular sieve having the framework structure isomorphous with zeolite beta and containing Si and Ti, but essentially no framework Al.
In spite of the success of the above-mentioned titanium silicates, there is a continuing need to synthesize highly active, selective, stable, and versatile heterogeneous olefin epoxidation catalysts in order to improve productivity and facilitate downstream separation.
According to the invention, it has now been found that a novel titanium dealuminated MCM-22-type material, prepared by the impregnation of a dealuminated MCM-22 with TiCl.sub.4, is a highly active olefin epoxidation catalyst which retains its catalytic activity after repeated uses. It is believed that this synthesis method isolates the titanium atoms inside the zeolite's supercages and surface pockets, thus preventing the formation of bulk TiO.sub.2 particles.